Los Angeles, Nov. 23: The Agatha Christie whodunnit The Mousetrap, by a mile the world’s longest-running play, will celebrate 50 years on the London stage on Monday with a grand party.

The 20,807th performance, on the anniversary of the opening, will be attended by the Queen and the Duke of Edinburgh — surprisingly they have never seen the murder mystery before — Lord Attenborough, who was in the first cast, and about a third of the 332 actors who have appeared in it.

Sir Stephen Waley-Cohen, the show’s producer, predicted that The Mousetrap could run forever. He told The Telegraph yesterday: “I don’t see why it should ever come off.

“It is still doing good business. It’s sold out on Fridays and Saturdays and though the audience is sometimes down to 50 per cent on cold, wet November nights earlier in the week, I don’t think there has been a week since it opened when it wasn’t in profit.”

David Turner, who has directed the play since 1987, travelling from Worthing to the West End every day, said: “People have sometimes said to me that we’re only keeping it on to break records. It’s nonsense, of course. There aren’t any records left to break.”

Christie’s murder mystery — after the climax the audience is always begged never to reveal whodidit to their friends — opened at the Ambassadors Theatre on November 25, 1952, when Winston Churchill was Prime Minister, Harry S Truman was President of the US, and television stopped at 10.30 pm. Quaintly old-fashioned, it has run without interruption ever since and has been seen by 10 million theatregoers.